Ties the radix trees and page maps into the VFS, KFS, and mmap(). Still
lots of little corner cases, it can't block yet, and I bet we're
screwing something up with page refcounting (which will cause memory
leaks).

Big note: this breaks support for appserver-managed files. It had to
happen sooner or later. The memory mapping currently only works for
files through do_mmap, since processes don't know about files yet.